SUMMARY OF RESULTS AND SUGGESTIONS. 161 



same exposure given by Prof. Michie Smith in his great 40-foot 

 telescope, whilst an exposure with the Cousens or with the 

 Dallmeyer instruments were equivalent respectively to 15 or 90 

 times that exposure with the Waters, and 30 or 180 times that 

 given by Dr. Michie Smith. 



Now as to results. The J-second exposure given with the 

 great 40-foot telescope " shows the very beginning of the eclipse 

 with a range of small prominences round nearly half the 

 sun's limb." The Waters 1-second plate, which was therefore 

 four times the one just mentioned, shows not merely the 

 prominences, but a distinct ring of corona, some two or three 

 minutes in height. We may take it, then, that if the object 

 be to secure the prominences alone with little or nothing of 

 the corona, it will not be safe to exceed greatly the exposure 

 of J second with//80, or ^ second with //1 5; the plate being 

 supposed of maximum sensitiveness, like the " W rat ten and 

 Wainwright drop shutter " employed by Dr. Michie Smith. 

 This exposure will be sufficient, and anything much longer will 

 be undesirable. 



The next plate in order of light efficiency is the first of the 

 Cousens series, and the estimated exposure was second, 

 equivalent to fifteen times that of the J-second exposure with 

 the Madras 40-foot lens, which we will take as our unit. Here 

 the exposure of the lower corona, up to 3' of arc from the limb, 

 is full. The prominences are seen, but are much over-exposed, 

 and the brightest begins to eat into the limb of the moon. 

 The fainter parts of the corona are also coming out distinctly, 

 and the roots of the three great coronal rays can be traced to a 

 full radius from the limb. 



The 5-second Waters, which corresponds to 20 units, and 

 therefore is ^ as efficient again as the Cousens first plate, 

 shows a very considerable development of the corona. The 

 next two Cousens plates and the third Waters have the values 

 of 30, 60 and 80 respectively, but do not greatly extend the 

 dimensions of the corona beyond that of the 5-second Waters. 

 Exposures, therefore, of from 20 to 30 units, or from ^ to J of a 

 second with //1 5, are sufficient to bring up satisfactorily nearly 

 the whole of the corona as ordinarily shown on photographs. 

 The gain in extent by increasing these exposures, with //1 5, to 

 1 second and 1J second, does not appear to be of any important 

 amount, whilst the risk of entirely losing the detail in the 

 lower corona by over-exposure increases rapidly with the pro- 

 longation of the time. 



The five splendid photographs obtained by Mr. Thwaites and 

 Mr. Fred Bacon fully confirm these conclusions. Mr. Thwaites 

 employed an o.g. of 4'25 inches aperture and 7.1 inches focus ; 

 Mr. Bacon had a similar aperture but a focal length of 60 inches. 

 The f/a was therefore 16'7 in the first case and 14f in the 



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